The present invention relates to a percussion drill and in particular to an improvement whereby the drill is free from undesirable impact which might occur after the workpiece is pierced.
A conventional percussion drill comprises a hollow cylinder in which a striker is slidably disposed to move forward under positive high pressure developed behind the striker in response to a forward movement of a piston. The striker hits the rear end of a slide shaft disposed in the forward end portion of the cylinder, the striker being bounced rearward and pulled to the original position under a negative pressure developed in response to a rearward movement of the piston. This process is repeated as long as the drill bit attached to the forward end of the shaft is working with a solid material. When the latter is pierced, the working load on the slide shaft is completely removed causing it to extend forward. This causes a compression to occur in the air chamber ahead of the striker and the latter is returned to the rear and again knocked forward in response to the forward movement of the piston in the next stroke. To prevent this knocking, a hole is provided in the cylinder wall to allow the compressed air to escape when the striker approaches the forward end of the cylinder. However, because of this hole, the mechanical energy of the striker is not completely consumed and undesirable impact results. More specifically, the cylinder is provided with an inner sleeve through which the slide shaft axially moves. When the workpiece is pierced, the striker tends to knock the end of the sleeve which would eventually result in breakage of the drill unit. Furthermore, in cases where the drill is held against a ceiling panel or the like, the striker is pulled by gravity and knocked forward again by forward stroke of the piston.